Self-calibrated adaptive equalization system and methods of performing the same

ABSTRACT

A self-calibrating, adaptive equalization system for generating an ideal digital signal is disclosed. The adaptive equalization system includes an equalizer and a high-gain buffer. The equalizer includes a first equalizer loop that feeds-back a control voltage to the equalizer and the high-gain buffer that includes a second equalizer loop that feeds-back a high-pass-to-low-pass filter ratio signal. Each of the first and second equalizer loops has a high-pass and a low-pass filter, rectifying circuits for each of the filters, and an integrating circuit that compares signal energy output from the rectifiers. The adaptive equalization system generates an ideal digital signal.

CROSS REFERENCE TO RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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BACKGROUND OF THE INVENTION

The non-ideal effects associated with channel loss in connection withbroadband data communication systems such as high-definition television(HDTV) impact signal quality increasingly as the bit rate increases. Inparticular, non-ideal effects such as skin effect loss and dielectricloss in the channel, e.g., cable, printed circuit board trace, and thelike, attenuate data more significantly at higher frequencies. Indeed,data attenuation can be represented by the following transfer function,L(f):

L(ƒ)=e ^(−1(ks√{square root over (iƒ)}+k) ^(d) ^(|ƒ|))   (1)

where f is the frequency, I is the channel length, and k_(s) and k_(d)are the skin effect loss constant and dielectric loss constant of thechannel, respectively.

One means for avoiding bit errors and inter-symbol interference (ISI)that results from the interference between adjacent pulses and forreceiving a high quality data signal is “equalization”. Equalizationcounteracts channel loss to compensate for transmission loss and torecover the distorted signal using an inverse or reciprocal frequencytransfer function as the channel loss, i.e., 1/L(f).

Because the exact characteristics of the channel are unknown, adaptiveequalization is preferable to fixed equalization. Adaptive equalizationrefers to the ability of the system to adapt to find the propercompensation level for a specific channel.

One such adaptive equalization system known to the relevant art is shownin FIG. 1. The equalization system 10 includes an equalizer 15 having anequalizer loop 18 to adjust the control voltage 11 of the equalizer 15to achieve an optimum equalization level. More particularly, theequalizer loop 18 includes a low-pass filter (LPF) 12, a high-passfilter (HPF) 14, a pair of rectifiers 17 and 19, and an integratingcircuit or comparator 13.

In operation, output from the equalizer 15 is sent to both the LPF 12and HPF 14, which extract signal energy within the respective frequencybands. Filter outputs are rectified by the pair of rectifiers 17 and 19and then compared by the integrating circuit 13, which determines thedifference between the signal energies of the LPF 12 and the HPF 14.

Based on that difference, the integrating circuit 13 outputs a controlsignal 11, e.g., a control voltage, to the equalizer 15. The equalizer15 then uses the control signal 11 to adjust the high-frequency gain ofthe equalizer 15. The equalizer feedback loop 18 continues to adjust thecontrol signal until the signal energy levels of the LPF 12 and HPF 14are equal, which is to say that the difference between the signalenergies is zero.

The ratio between the signal energies of the LPF 12 and the HPF 14 ispreset and fixed, e.g., the ratio of high-pass-to-low-pass filter signalenergy can be preset and fixed at 1:1. However, in practice, the adaptedoperating point is not fixed so the high-pass-to-low-pass filter signalenergy, typically, is not 1:1. The high-pass-to-low-pass filter ratio isvariable due to, for example, the channeling medium, the transmitteddata, process, supply voltage, temperature, and the like. Accordingly,the control signal 11 of the adaptive equalizer 15 may be imperfect,resulting in a correspondingly incorrect or non-ideal equalizer gainsetting.

In either instance, over-equalizing or under-equalizing an attenuatedinput signal causes jitter. Consequently, it would be desirable toprovide a self-calibrating adaptive equalization system to improvejitter performance.

BRIEF SUMMARY OF THE INVENTION

A self-calibrating, adaptive equalization system for generating an idealdigital signal is disclosed. The adaptive equalization system includesan equalizer and a high-gain buffer or “slicer”, that is adapted toprovide a sinc²(x) spectrum. The equalizer includes a first equalizer(feedback) loop that feeds-back a control signal to the equalizer. Thehigh-gain buffer includes a second equalizer (feedback) loop thatprovides a high-pass-to-low-pass filter ratio signal, which is fed-backto the first equalizer loop to adjust the control signal.

Each of the first and second equalizer loops has a high-pass and alow-pass filter, rectifying circuits for each of the filters, and anintegrating circuit that compares signal energy output from therectifiers. The adaptive equalization system generates an ideal digitalsignal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be more fully understood by reference to thefollowing Detailed Description of the invention in conjunction with theDrawings, of which:

FIG. 1 shows an illustrative block diagram of an adaptive equalizer of atype known to the prior art;

FIG. 2 shows an illustrative block diagram of an adaptive equalizer inaccordance with the present invention;

FIG. 3 shows high-gain buffer output voltage waveforms in the timedomain; and

FIG. 4 shows corrective signal output.

DETAILED DESCRIPTION OF THE INVENTION

An illustrative block diagram of an adaptive equalization system inaccordance with the present invention is shown in FIG. 2. The disclosedadaptive equalization system 20 includes an adaptive equalizer 15 andequalizer (feedback) loop 18, in combination with a high-gain buffer (or“slicer”) 25. The adaptive equalizer 15 and equalizer (feedback) loop 18are similar to those known to the prior art. The high-gain buffer 25includes a second equalizer (feedback) loop 28, the composition of whichis identical or virtually identical to the first equalizer loop 18.

As previously described, the adaptive equalizer 15 receives andprocesses an attenuated input signal that can be affected by thetransmitted data, the process, the temperature, the supply voltage, andthe like. Output from the adaptive equalizer 15 is then fed to theadaptive equalizer loop 18. The LPF 12 and HPF 14 of the adaptiveequalizer loop 18 extract signal energy within the respective frequencybands of the equalizer output. The outputs from the filters 12 and 14are then rectified by their respective rectifiers 17 and 19 and therectified signals are compared by the integrating circuit 13.

The integrating circuit 13 determines the difference between the signalenergies of the LPF 12 and the HPF 14. As will be discussed in greaterdetail below, to account for changes in the high-pass-to-low-pass filterratio (HPF/LPF) due to, for example, the transmitted data, the process,supply voltage, temperature, and the like, an “alpha” correction signalwill be used to account for the changes and, as a result, to adjust thecontrol signal 11 to the adaptive equalizer 15.

FIG. 3 shows illustrative high-gain buffer voltage output waveforms inthe time domain. In the time domain, the high-gain buffer 25 creates awaveform having a very sharp edge rate, which is to say that the riseand fall times are shorter and more abrupt, and also having a fastertransition.

In the frequency domain, however, the high-gain buffer 25 outputs anideal digital signal having sinc²(x) spectrum characteristics, which isto say that the edges of the waveform have been squared-up. Squaring-upthe edges of the waveform translate into faster transition times andshorter rise and fall times. As a result, the frequency spectrum, orfrequency content, is representative of the desired output after theadaptive equalization stage.

To take advantage of this, the high-gain buffer 25 is used to generate acorrective, “alpha” signal as a reference signal for use by the adaptiveequalizer 15 in generating an ideal digital signal. For example, thehigh-gain buffer 25 output is fed through each of a LPF 22 and a HPF 24in a second equalizer loop 28. The LPF 22 and HPF 24 of the secondequalizer loop 28 also extract signal energy within the respectivefrequency bands of the high-gain buffer 25 output signal. The signalenergies from the filters 22 and 24 are then rectified by theirrespective rectifiers 27 and 29. The rectified signals are compared bythe integrating circuit 23.

The integrating circuit 23 determines the difference between the signalenergies of the LPF 22 and the HPF 24. Based on the signal energydifference, the integrating circuit 23 outputs a corrective, “alpha”signal 21. See FIG. 4. The corrective, “alpha” signal 21 is fed-back toeach of the HPFs 14 and 24 of the first and second equalizer loops 18and 28, respectively. Alternatively, the corrective, “alpha” signal 21can be fed-back to each of the LPFs 12 and 22 of the first and secondequalizer loops 18 and 28, respectively, or to the HPFs and the LPFs ofboth loops.

The corrective, “alpha” signal 21 establishes the target HPF/LPF ratiogain, ensuring that the signal energy differences between the LPF 12 andHPF 14 are driven to zero (and the HPF/LPF ratio is driven to unity).After taking into account the HPF/LPF ratio, the first equalizer loop 18generates the ideal control signal 11.

Output from the high-gain buffer 25 continues to pass through the LPF 22and through the HPF 24 where the signal energy is adjusted as a functionof the corrective, “alpha” signal 21. This corrective process continuesuntil the signal energy levels between the LPF 22 and HPF 24 are equal,which is to say that the difference between the signal energies is zero.The resulting corrective, “alpha” signal 21 associated with the secondequalizer loop 28 is representative of the HPF/LPF ratio required tomake the output of the entire system 20 “ideal”.

Using a SPICE simulation tool, a fixed “alpha” equalization system 10and a self-calibrating equalization system 20 were simulated. Table Isummarizes the jitter results for a High-Definition Multimedia InterfaceChannel 5M cable (2.5 Gbps) for each system. NNN refers to a nominalprocess, a nominal supply voltage, and a nominal, i.e., room,temperature. HLH refers to a strong process corner, a low supplyvoltage, and a high temperature. LHL refers to a weak process corner, ahigh supply voltage, and a low temperature.

As shown in Table I, jitter of the self-calibrating adaptiveequalization system 20 is improved (reduced) by about one-third over thefixed energy-ratio system for the HLH and LHL simulations.

TABLE I Type of “Alpha” SELF- FIXED CALIBRATING ALPHA ALPHA Corner (PVT)NNN HLH LHL NNN HLH LHL Alpha (mV) 650 650 650 650 750 640 Jitter (ps)60 80 70 60 56 57

In summary, the high-gain buffer 25 and second equalizer loop 28 areadapted to determine an optimum HPF/LPF ratio that is self-calibratingand, moreover, independent of process, supply voltage, and temperatureconditions.

It will be apparent to those skilled in the art that modifications toand variations of the disclosed method and system are possible withoutdeparting from the inventive concepts disclosed herein, and thereforethe invention should not be viewed as limited except to the full scopeand spirit of the appended claims.

1. A self-calibrating, adaptive equalization system for generating anideal digital signal, the adaptive equalization system comprising: anequalizer for receiving an input signal, the equalizer having a firstequalizer loop that includes a low-pass filter and a high-pass filterfor generating a control signal; a high-gain buffer that is incommunication with, so as to receive an output from, the equalizer, thebuffer having a second equalizer loop that includes a low-pass filterand a high-pass filter, for generating a corrective signal; and meansfor comparing signal energies output from said low-pass filter and saidhigh-pass filter for each of the first and second equalizer loop, forgenerating the corrective signal and the control signal.
 2. The adaptiveequalization system as recited in claim 1, wherein said ideal digitalsignal is generated when there is no signal energy difference betweenthe low-pass filter and the high-pass filter of said first equalizerloop and there is no signal energy difference between the low-passfilter and the high-pass filter of said second equalizer loop.
 3. Theadaptive equalization system as recited in claim 1, wherein thecorrective signal corresponds to a high-pass filter-to-low-pass filtergain.
 4. The adaptive equalization system as recited in claim 1, whereinthe ideal digital signal has sinc²(x), frequency domain characteristics.5. The adaptive equalization system as recited in claim 1, wherein thecorrective signal is applied to one of the low-pass filter and thehigh-pass filter of the first equalizer loop, to account for a high-passfilter-to-low-pass filter gain.
 6. The adaptive equalization system asrecited in claim 1, wherein the corrective signal is applied to one ofthe low-pass filter and the high-pass filter of the second equalizerloop, to account for a high-pass filter-to-low-pass filter gain.
 7. Theadaptive equalization system as recited in claim 3, wherein the idealdigital signal has sinc²(x), frequency domain characteristics.
 8. Aself-calibrating, adaptive equalization system for generating an idealdigital signal, the adaptive equalization system comprising: anequalizer for receiving an input signal, the equalizer having a firstequalizer loop that includes a low-pass filter, a high-pass filter, andan integrating circuit that compares a signal energy output from saidlow-pass filter to a signal energy output of said high-pass filter andthat generates a control signal back to the equalizer; and a high-gainbuffer that is in communication with the equalizer for generating acorrective signal, the buffer having a second equalizer loop thatincludes: a low-pass filter, a high-pass filter, and an integratingcircuit for comparing a signal energy output from said low-pass filterto a signal energy output of said high-pass filter to generate acorrective signal.
 9. The adaptive equalization system as recited inclaim 8, wherein said ideal digital signal is generated when there is nosignal energy difference between the low-pass filter and the high-passfilter of said first equalizer loop and there is no signal energydifference between the low-pass filter and the high-pass filter of saidsecond first equalizer loop.
 10. The adaptive equalization system asrecited in claim 8, wherein the corrective signal corresponds to ahigh-pass filter-to-low-pass filter gain.
 11. The adaptive equalizationsystem as recited in claim 8, wherein the ideal digital signal hassinc²(x), frequency domain characteristics.
 12. The adaptiveequalization system as recited in claim 8, wherein the corrective signalis applied to one of the low-pass filter and the high-pass filter of thefirst equalizer loop, to account for a high-pass filter-to-low-passfilter gain.
 13. The adaptive equalization system as recited in claim 8,wherein the corrective signal is applied to one of the low-pass filterand the high-pass filter of the second equalizer loop, to account for ahigh-pass filter-to-low-pass filter gain.
 14. The adaptive equalizationsystem as recited in claim 8, wherein the ideal digital signal hassinc²(x), frequency domain characteristics.
 15. The adaptiveequalization system as recited in claim 8, wherein each of the low-passand the high-pass filters for each of the first and second equalizerloops includes a rectifying circuit.
 16. A method for generating anideal digital signal from a high-frequency input signal by adaptiveequalization, the method comprising: introducing the input signalthrough an adaptive equalizer having a first equalizer loop, the firstequalizer loop including a low-pass filter and a high-pass filter;feeding an output signal from the equalizer to the first equalizer loopand to a high-gain buffer, the high-gain buffer having a secondequalizer loop that includes a low-pass filter and a high-pass filter;comparing signal energy output from the low-pass filter of the firstequalizer loop to signal energy output from the high-pass filter of thefirst equalizer loop, to determine a first difference in signal energy;generating a control signal based on the first difference; applying thecontrol signal to the adaptive equalizer; feeding an output signal fromthe high-gain buffer to the second equalizer loop; comparing the signalenergy output from the low-pass filter of the second equalizer loop tothe signal energy output from the high-pass filter of the secondequalizer loop, to determine a second difference in signal energy;generating a corrective signal based on the second difference; andapplying the corrective signal to one of the low-pass filter and thehigh-pass filter of the first equalizer loop and to one of the low-passfilter and the high-pass filter of the second equalizer loop; andadjusting the control signal to account for the corrective signal. 17.The method as recited in claim 16, wherein generating a correctivesignal includes generating a high-pass-to-low-pass filter ratio gainbased on the second difference.
 18. A method for generating an idealdigital signal from a high-frequency input signal by adaptiveequalization, the method comprising: introducing the input signalthrough an adaptive equalizer having a first equalizer loop, the firstequalizer loop including a low-pass filter and a high-pass filter;feeding an output signal from the equalizer to the first equalizer loopand to a high-gain buffer, the high-gain buffer having a secondequalizer loop that includes a low-pass filter and a high-pass filter;rectifying signal energy output from the low-pass and the high-passfilters of the first equalizer loop; comparing the rectified signalenergy output from the low-pass filter of the first equalizer loop tothe rectified signal energy output from the high-pass filter of thefirst equalizer loop, to determine a first difference in signal energy;generating a control voltage based on the first difference; applying thecontrol voltage to the adaptive equalizer; feeding an output signal fromthe high-gain buffer to the second equalizer loop; rectifying signalenergy output from the low-pass and the high-pass filters of the secondequalizer loop; comparing the rectified signal energy output from thelow-pass filter of the second equalizer loop to the rectified signalenergy output from the high-pass filter of the second equalizer loop, todetermine a second difference in signal energy; generating ahigh-pass-to-low-pass filter ratio gain based on the second difference;and applying the high-pass-to-low-pass filter ratio gain to one of thelow-pass filter and the high-pass filter of the first equalizer loop andto one of the low-pass filter and the high-pass filter of the secondequalizer loop; and adjusting the control voltage to account for thehigh-pass filter-to-low-pas filter ratio gain.